1. Field of the Invention
The present invention generally relates to anodizing of aluminum or aluminum alloys and is particularly concerned with a process for forming oxide films of high durability over the surface of aluminum at a high anodizing rate.
2. Description of the Prior Art
In the anodizing of aluminum, it is customary to hang the articles to be treated on racking devices made of aluminum in order to soak them in a solution containing about 15% of sulfuric acid. In such a method, the film of Al.sub.2 O.sub.3 formed over the surface of the aluminum by anodizing at a current density of about 1 A/dm.sup.2 while keeping the bath temperature at about 20.degree..+-.2.degree. C. for about 60 minutes has a thickness of about 15.mu.. The rate of film formation is about 0.25.mu./min. However, such a method hardly yields a film having a thickness of more than 20.mu., while thicknesses of more than 20.mu. are quite advantageous and desired in order to enhance the rust-resistant property of the anodized film against the increasing air pollution problems of the present day. Therefore, it is clear that the conventional methods of the prior art are not suited for the anodizing operations of the future.
Although an increase in the current density may result in a concomitant increase in the rate of film formation in view of the common knowledge in the field of electrochemistry, it inevitably entails a proportionate decrease in the amount (number of units) of articles which can be treated in one batch with a power source of the same capacity and an increase in the heat generated in accordance with Joule's law which in turn means a requirement for special apparatus for cooling. Local and uneven high temperatures which may cause "burning" or scorching should particularly be avoided.
Even though the disadvantages concerning the limited amount of articles to be treated and special cooling problems might be tolerated, sparking which may cause said burning problems should at least be avoided. However, the articles to be treated will inevitably spark above a certain point of critical voltage when the bath voltage rises in view of the increasing current density.
A rise in the bath temperature is the other measure for increasing the current density because it means a concurrent decrease in the electrical resistivity of the electrolyte solution. However, this measure has an obvious limitation (20.degree. C. at the most in the conventional operation) because it entails an increase in the rate of dissolution of the base or substrate metal into the solution, thereby reducing the thickness of the formed oxide film which also may become coarse and excessively porous at higher temperatures.
As previously described, the factors involved in and dominating the film forming rate are so interwoven with each other that a theoretical derivation of the desired conditions is very difficult if not impossible and an empirical mode of thinking has prevailed in most cases.
The above scrutiny with respect to the forming conditions of conventional soft anodized films may likewise be applied to those of hard anodized films. The "Tomaschov" method and the "Martin" method have hitherto been considered to be the most advanced methods for the formation of hard films. In the Tomaschov method, a 20% sulfuric acid bath is maintained at 2.degree..+-.1.degree. C. to perform the anodizing operation under the conditions of a current density of 2-5 A/dm.sup.2 obtained by a terminal voltage of 23-120 V for 4 hours. The thickness of the formed film is about 200.mu., wherein the rate of film formation is about 0.83.mu./min. In the Martin method, an anodizing operation under the conditions of a 15% sulfuric acid concentration, a bath temperature of 0.degree. C. and a current density of 2-2.5 A/dm.sup.2 by means of a terminal voltage of 25-60 V for 40 minutes gives a film having a thickness of 25.mu., wherein the rate of film formation is about 0.64.mu./min. Either one of these methods is very difficult to perform in a routine operation as compared with a conventional hard film anodization, because of the need for a means for maintaining a very low temperature in the stated very narrow range.
As a result of a systematic investigation on various factors which may influence the film forming rate, the present inventor has found that although the prime importance must be placed on the increase in the current density, a sufficient consideration should also be extended to the material of the racking device which holds the articles to be treated on the anode, and to the cooling of the electrolyte solution to prevent the disadvantages attributable to the poor conducting property and to the heat generated in compliance with Joule's law, sparking problems and the like.
Cooling of the electrolyte solution by agitation with a mechanical stirrer or bubbling air, which is customary for keeping the bath temperature constant and uniform, has been found to be unsatisfactory because neither one of these methods is effective for preventing convection which might form graduated layers of liquids having different temperatures which tend to cause local heating.